1. Field of the Invention
This invention relates to a process for producing a glass preform with minimum surface roughening and which has a minimum of impurities, residual water and air bubbles, and more particularly, to a process for producing a glass preform which contains not more than 0.1 ppm of residual water.
2. Description of the Prior Art
The VAD process that produces a glass preform for optical fiber by hydrolyzing halides of Si, Ge, B, P, etc., in an oxygen-hydrogen flame is considered effective for producing a silica-containing glass preform that is substantially free of impurities, particularly, transition metals such as Fe. The process is advantageous for preparing a preform for a cheap low-loss optical fiber having a desired distribution of refractive index in the radial direction and a uniform composition in both circumferential and longitudinal directions. The outline of the VAD process is as follows: Fine glass particles formed by flame hydrolysis and distributed within the flame according to their radial distance from the center are deposited in the axial direction on a rotating starting member such as a glass plate or rod to form a layer of glass soot in a cylindrical form which is then sintered to form a transparent preform. Among the advantages of this process are high yield, high purity (except for the residual OH), short production time (less than half that of other processes), easy control of refractive index distribution, and the very few steps required. Therefore, the process is considered suitable for quantity production and hence has a very great industrial value. However, the VAD process that relies on a hydrolytic reaction cannot be performed without having about 30 to 70 ppm of residual water that derives from part of the unreacted water.
In optical communications technology, that is an increasing demand to use waveforms in the vicinity of 1.3 .mu.m where absorption loss due to imcomplete structure of an optical fiber is minimum. But this is also a range in which absorption loss due to the presence of residual OH groups in the fiber is great. It is necessary to reduce the residual OH content of the fiber to less than 0.3 ppm in order to use this particular wavelength range. For this purpose, various physical or chemical treatments have been employed. Among physical methods of eliminating OH groups are hot air drying and vacuum drying. A known chemical method of eliminating OH groups uses a gaseous halide to decompose water in an aggregate of fine glass particles (to form a glass preform) into hydrogen halide and oxygen. It is difficult to reduce the residual OH content to less than 0.1 ppm by the physical method. The chemical method can only reduce the OH level to about 1 ppm even after a heat treatment at a temperature higher than 800.degree. C., and an OH level on the order of 0.1 ppm is not achieved. Since the chemical method relies on HF, Cl.sub.2, SOCl.sub.2 gas and the like, the surface of the aggregate may be etched depending on the vitrifying conditions, and this can cause increased light loss due to the rough interface of the fiber. In addition, unless the concentration of the gaseous halogens or halide, the flow of carrier gas, and the temperature are properly selected, cracking occurs in the surface of the aggregate making it unsuitable for producing a fiber preform. Another problem is that transition metals which increase absorption loss or alkali or alkaline earth metals which reduce fiber strength may enter the fine glass particles in the halogen gas atmosphere through the tube in the center of the furnace.